1. Field of the Invention
The present invention relates to a transparent film for forming a toner image, and a process for forming a toner image. More specifically, the present invention relates to a process for forming a toner image in accordance with an electrophotographic process.
2. Related Background Art
In the general process for forming a full-color image, a full-color image has heretofore been formed in the following manner. A magenta toner image is formed by uniformly charging a photosensitive member of a photosensitive drum, conducting image exposure with a laser beam modulated by magenta image signals of an original document to form an electrostatic latent image on the photosensitive drum, developing the electrostatic latent image by a magenta developing device to form a magenta toner image on the photosensitive drum, and transferring the magenta toner image developed on the photosensitive drum by means of a charger for transfer to a recording medium that has been fed.
After completion of the above-described development and transfer, the photosensitive drum is then subjected to static charge elimination by a charger for static charge elimination, cleaned and again charged by a primary charger to conduct the formation of a cyan toner image on the photosensitive drum and the transfer of the cyan toner image to the recording medium, to which the magenta toner image has been transferred, in the same manner as described above. Further, this process is successively conducted as to a yellow color and a black color to transfer the toner images of the four colors to the recording medium. Furthermore, the toner images of the four colors are fixed to the recording medium by the action of heat and pressure by a fixing means such as a fixing roller, thereby forming a full-color image.
In recent years, such image forming apparatus have not been limited to use only as copying machines for office management for copying so-called original documents, and have begun to be used in the fields of printers as output devices for computers and personal copying for individual use. Besides the field typified by such a laser beam printer, the development of facsimiles for plain paper making use of the basic technology is also rapidly growing.
Therefore, in such an image forming apparatus as described above, smaller size, lighter weight, higher speed and higher image quality, and higher reliability have been pursued, and so the apparatus has come to be constructed by simpler elements at various points. As a result, toners have also been required to have higher performance. Therefore, an excellent image forming apparatus has not come to be realized unless improvement in the performance of the toners has been achieved.
In keeping with the various needs of copying in recent years, the demand for color copying has rapidly increased. In order to more faithfully copy an original color image, still higher image quality and higher resolution have been desired. From these points of view, toners used in a color-image forming process are required to be good in melting ability and color-mixing ability upon application of heat, and there is hence a demand for development of toners high in sharply melting ability, which have a low softening point and a low melting temperature. The use of such toners high in sharply melting ability enables widening the color tone reproducibility range of copies and is able to obtain color copies faithful to the original color image.
However, such toners high in sharply melting ability generally have high affinity for a fixing roller and tend to offset to the fixing roller. In particular, in the case of a fixing means in a color-image forming apparatus, there is a tendency to easily cause offset due to increase in the thickness of toner layers by the formation of a plurality of toner layers composed of magenta toner, cyan toner, yellow toner and black toner on a recording medium.
In order to improve the releasability of toners from a fixing roller, it has heretofore been conducted to, for example, form the surface of the fixing roller with a material having excellent release property (for example, silicone rubber or fluororesin) for the toners and further coat the surface with a thin film of a liquid having high release property, such as silicone oil or fluorine-containing oil to prevent offset and the fatigue of the roller surface. This method is extremely effective in that the offset of toners is prevented, but involves problems that a device for feeding an anti-offset liquid is required to complicate the fixing device and that the oil applied causes delamination or peeling between layers making up the fixing roller to consequently facilitate shortened life of the fixing roller.
In keeping with the various needs of copying in recent years, various kinds of paper, coated paper and plastic films have been used as recording mediums. In particular, attention is paid to the necessity of light-transmitting sheets (OHP sheets) for utilizing an over-head projector (OHP) for presentation. The OHP sheet has low oil-absorbing capability, unlike paper, so that the oil used in the above-described fixing device adheres to the surface of the OHP sheet. As a result, the OHP sheet, on which an image has been formed, has been unable to avoid having a sticky feel due to the adhesion of the oil and incurs deterioration of the image quality. In addition, there is a possibility that a releasing oil such as silicone oil may be evaporated by heat to contaminate the interior of the fixing device, or that problems such as treatment of recovered oil may arise.
Accordingly, it is greatly expected to establish a fixing system which can solve the above-described problems and does not require oil application upon the fixing of an image, and to develop novel toners for that purpose.
In order to achieve the above subject, Japanese Patent Application Laid-Open No. 61-273554 discloses a toner comprising a release agent such as a wax. In case of the toner containing a wax, the thermal conductivity of the toner is enhanced by virtue of the wax which melts at a lower temperature. As a result, the fixing at a low temperature is feasible. More preferably, the wax melted upon fixing is also able to act as a release agent, and so offset at a high temperature can be prevented without applying a release agent such as an oil to the fixing roller.
When a color toner image or full-color toner image is formed on a light-transmitting sheet using an electrophotographic system according to a dry developing process and the image is projected by means of an OHP apparatus, there is presented a phenomenon that the projected image exhibits a grayish tone as a whole though the full-color image on the light-transmitting sheet shows sufficient coloring property, and so the range of color tone reproduction becomes very narrow. This phenomenon presents itself for the following reason. Namely, since the unfixed toner image formed on the light-transmitting sheet with a smooth surface is not fully made fluid even by heating upon image fixing and retains graininess, incident light is scattered upon projection to cast a shadow on the screen. In a middle tone area or highlight area of image, at which the image density is low, in particular, the absorption by the dye or pigment in the toner of each color is lowered due to the decreased number of toner particles. Therefore, the color tone to be reproduced becomes grayish.
On the other hand, when a toner image formed on a recording medium such as plain paper is visually observed, an reflected image of light struck on the toner image is visually observed. Therefore, the image quality is scarcely influenced even when some graininess is left on the toner surface. When the toner image is observed or projected on a screen as transmitted light like OHP, however, the light transmission property is deteriorated due to the scattering of light if the shape of the toner particles is clearly retained, so that the tone becomes grayish. Accordingly, a recording medium used in OHP is required to have an effect of reducing the graininess of toners after the fixing of a color image to improve light transmission property.
Therefore, various light-transmitting recording mediums provided with a surface layer formed of a thermoplastic resin such as a styrene-acrylic resin or polyester resin on their transparent base sheets have heretofore been proposed as recording mediums for electrophotography from the viewpoints of improvement in the clarity or sharpness of images due to the enhancement of the fixing ability of toners and improvement in the conveyability and blocking resistance of the recording medium. For example, Japanese Patent Application Laid-Open Nos. 1-263085, 6-19180, 6-19485 and 6-332221 may be mentioned.
As a means for reducing the graininess of toners after the fixing to improve light transmission property, there is used a method in which toner particles are embedded into a surface layer by heat and pressure upon the fixing, as described in Japanese Patent Application Laid-Open Nos. 2-263642 and 7-199515. In these light-transmitting recording mediums, the graininess of the toners after the fixing is improved by the effect of the resin forming the surface layer. Therefore, the light transmission property is improved, and so the projectability in OHP becomes excellent. When a resin, which is not fully plasticized by heat and pressure upon the fixing, is used in the surface layer, however, the penetration of toner particles into the receiving layer, or the surface layer is extremely reduced, and so the projected image comes to exhibit a grayish tone.
All the above-described recording mediums have been those used in the case where a toner image is fixed by using a release agent such as an oil for a fixing roller. Namely, as to the OHP sheets mentioned above, an oilless fixing process, in which a wax is contained as a release agent in a toner and no release agent such as oil is applied to the surface of the fixing roller, is not taken into consideration. Therefore, when such toner as described above is used to heat and fix a toner image with a small amount of toner, the image area proportion of which is only about 5%, to the OHP sheet, the offset resistance is satisfactory at the toner image area by virtue of the action of the wax as a release agent in the toner, but the action of the wax as a release agent becomes insufficient at an area on which the toner image is not formed over a wide range to tend to bring about such a phenomenon that the surface layer formed of the thermoplastic resin sticks to the fixing roller. Accordingly, there is a demand for improvement in the recording mediums so as to be suited to the oilless fixing process making use of the above-mentioned toner.
Japanese Patent Application Laid-Open No. 5-181300 describes that a toner containing a wax component is heated and fixed to a transparent recording medium by using the oilless fixing process in which the surface of the fixing roller is not coated with a release agent such as an oil. However, this document does not describe anything about the fact that a toner image with a little toner having an image area proportion of less than 5% is fixed.
Japanese Patent Application Laid-Open No. 9-218527 has proposed that a release agent of wax type is used to prevent a recording medium from sticking to a fixing roller. According to this proposal, a release agent having a clear melting point is contained in a thermoplastic resin. In this case, the wax is melted upon fixing to impart releasability to the recording medium. When the thermoplastic resin has a glass transition temperature (Tg) comparable to that of toners, however, sticking of the thermoplastic resin to the fixing roller may occur in some cases before the wax is melted. In addition, in the case of the resin containing wax, its transparency becomes insufficient by any means, so that the coloring property of the resulting color image may become insufficient in some cases when it is projected by OHP.
It is an object of the present invention to provide a transparent film for forming a toner image, which does not cause sticking to a fixing roller.
Another object of the present invention is to provide a process for forming a transparent color image exhibiting color reproductivity of middle tone and excellent coloring property when projected by OHP and not interfering with a fixing roller by using the transparent film for forming a toner image.
The above objects can be achieved by the present invention described below.
According to the present invention, there is thus provided a transparent film for forming a toner image, comprising a transparent base material and a resin layer provided on the surface thereof, wherein the resin layer has a release oil.
According to the present invention, there is also provided a process for forming a toner image, comprising the steps of forming a toner image on a transparent film comprising a transparent base material and a resin layer provided on the surface thereof and fixing the toner image by heating and pressurizing it, wherein the resin layer has a release oil.
In the transparent film for forming a toner image according to the present invention, the release oil is present in or on the resin layer on which a toner image will be formed. Therefore, the release oil can directly act on a fixing means such as a fixing roller upon fixing images by heating and pressurizing, or heat-and-pressure fixing of images to prevent the transparent film from sticking to the fixing means. Since the release oil is liquid unlike waxes even when it is not melted, the effect of preventing the sticking of the transparent film to the fixing means can be developed promptly.